The present invention relates in general to a shift-by-wire transmission, and, more specifically, to a bias spring operable in a default-to-park mechanism.
In a shift-by-wire transmission arrangement, the traditional mechanical connection between a transmission gear selector and the transmission is eliminated. Instead, a transmission control device transmits an electrical signal to an electronic controller which directs separate actuators to apply or release the various friction elements of the transmission to obtain a desired gear ratio. The control device is no longer necessarily in the form of a lever because the control device is no longer moving a mechanical connection for controlling the transmission. Instead, the control device is typically an electro-mechanical interface (e.g., a series of buttons, lever, or knob) that is used to instruct the transmission to switch between the transmission ranges.
In order to ensure that a vehicle enters an immobile state when a shift-by-wire system enters a Park state, a failsafe system may be implemented known as a default-to-park system. In some embodiments, the transmission may be provided with a park pawl operated by an actuator in order to selectably engage with at least one gear in the transmission to inhibit vehicle motion or to release from the at least one gear to permit vehicle motion. The park pawl may be engaged in response to a driver shifting the transmission gear selector to PARK.
The actuator for a default-to-park system may be a hydraulic actuator (i.e., valve), for example. Under normal vehicle operation, an engine drives a transmission pump to supply hydraulic pressure to the actuator and enable application or release of the park pawl. When the engine is turned off or in the event of certain component failures, the hydraulic pressure is lost. A return element such as a bias spring is used to automatically return the park pawl into engagement with the transmission to act as a transmission brake or lock so that vehicle movement is halted by default under those conditions.
The bias spring is typically comprised of a torsion spring, as shown in U.S. Patent Application Publication US2015/0308571, published Oct. 29, 2015, entitled “Parking Override Device for a Shift-By-Wire Transmission,” which is incorporated herein by reference in its entirety. Torsion springs are used in many applications where package space or other considerations may preclude the use of an extension or compression spring. One potential problem with torsion springs in the known configurations is the tendency for the ends where the spring is anchored (to either a stationary or a moving component) to “tilt” as the spring is loaded (i.e., wound-up) by motion of the components. If great enough, this tilt can cause the spring end to become disconnected from the component it is intended to remain in contact with. This tendency to tilt is usually countered by increasing the wire diameter of the spring itself. However, increasing the wire diameter has the effect of increasing the rate of the spring, so that as the spring is deflected the spring force rapidly increases. This increase in force from the spring is often undesirable. The desired state for the spring as used in the default-to-park mechanism is to provide an initial loading force (i.e., an “installed force”) when the components are in their resting state (to keep the park pawl engaged), with the force increasing as little as possible as the spring is deflected during operation of the components so that the capacity (and cost) of the actuator can be kept low.